“There is some evidence suggesting that mild to moderate alcohol consumption can protect against diseases like Alzheimer's in humans,” said During. “But it's not apparent how this happens.”

“Doses of alcohol moderately improved the animals' ability to remember a negative event,” During said. “The research also suggests that high levels of alcohol can reinforce bad memories. “People who drink to forget bad memories may actually be doing the opposite by reinforcing the neural circuits that control negative emotional memory."

He and his colleague, Margaret Kalev-Zylinska, a postdoctoral researcher at the University of Auckland, in New Zealand, uncovered a neuronal mechanism that may help explain the link between alcohol and improved memory.

“We saw a noticeable change on the surface of certain neurons in rats that were given alcohol,” During said. “This change may have something to do with the positive effects of alcohol on memory.”

During and Kalev-Zylinska designed a special liquid diet for the rats. One formulation included a low dose of alcohol, comparable to two or three drinks a day for a human, while the other diet included a much higher dose of alcohol, comparable to six or seven drinks a day for a human. A third group of rats was given a liquid diet without alcohol. All animals were given their respective diets daily for about four weeks.

The researchers measured the rats' blood-alcohol levels three times throughout the study. Toward the end of the study, they subjected the rats to two different memory tasks.

For the first task, the rats were given several minutes to examine two identical, square plastic objects. After a certain amount of time, a researcher replaced one of the objects with a new, round object made of glass. The researchers measured the amount of time that each rat spent checking out the new object – an indication that the animal recognizes it as a new object.

Rats given low doses of alcohol spent about three times longer examining the new object than did rats on the alcohol-free diet. Rats given the high dose of alcohol spent equivalent amounts of time checking out both objects, suggesting that they were unable to differentiate the old object from the new one.

For the second task rats were placed in a box with two chambers separated by a door. One chamber was well-lit, while the adjacent chamber was dark. After placing a rat in the well-lit chamber and then lifting the door, the researchers timed how quickly the rats entered the dark chamber (rats are nocturnal, and naturally prefer dark spaces.) Once inside the dark chamber, the rat received a mild electric shock to its feet.

The researchers repeated this same experiment 24 hours later, and kept track of how long it took the animal to enter the dark chamber. Many of the animals re-entered the dark area, yet the rats given alcohol waited anywhere from 2.5 to 4.5 times longer to enter the dark chamber than did the animals given the alcohol-free diet.

“The results suggest that both doses of alcohol moderately improved the animals' ability to remember this negative event, since they seemed hesitant to go into the dark area,” During said. “It also suggests that high levels of alcohol can reinforce bad memories.

“People who drink to forget bad memories may actually be doing the opposite by reinforcing the neural circuits that control negative emotional memory,” he continued.

At the end of the study, the researchers analyzed brain and liver tissue from each animal.

They found that low levels of alcohol increased the expression of a particular receptor, NR1, on the surface of neurons in a region of the brain, the hippocampus, that plays a role in memory. Researchers think that NR1 plays a role in memory and learning.

In a separate set of experiments, During and Kalev-Zylinska increased the number of NR1 receptors in another group of rats, and found that this boost improved the animals' memories to an extent similar to the improvement seen in the rats given low doses of alcohol. They also they used a new gene transfer technique to knock down the NR1 receptors in a group of rats given alcohol – alcohol had no memory-enhancing effects on these animals.

“These experiments suggest that the effect of alcohol works through the NR1 receptor, at least where memory and learning are concerned,” During said.

“We didn't see any toxic effects of low-level alcohol consumption on the brain or the liver,” During said. “It didn't damage neurons nor did it cause liver damage during the short study. But the higher dose of alcohol damaged both.”